In the Feb. 15 issue of CMAJ, the lead editorial1 and a commentary by Sari Kovats and Andrew Haines2 refer to “global average surface air temperature” and “global average surface temperature” respectively. It appears that this is a key indicator upon which inferences about “global warming” are based. Yet I have not been able to find an adequate description of the method by which this measure is obtained. Even as a nonclimatologist, I recognize that temperature varies by location, altitude, season, hour of the day and numerous other factors. The concept of an “average global temperature” is therefore very difficult to grasp.

Are there internationally agreed-upon sampling sites, sampling times and sampling procedures for such calculations? Have these been in place for decades and applied consistently to allow us to comment on global historical trends, let alone make projections for the future? Do we know the 95% confidence intervals around these average temperatures? Until I have more information about how this critical indicator is measured, I cannot evaluate the “evidence” for an impact on health.